Anodizing is a well known surface treatment carried out on articles made of (or coated with) aluminum or anodizable aluminum alloys for the purpose of improving the decorative appeal of the articles and/or for improving surface durability. The procedure involves electrolysis carried out in an electrolyte containing a strong acid, such as sulphuric acid, phosphoric acid, oxalic acid or the like, using the aluminum article as an anode. As the electrolysis proceeds, an anodic film of aluminum oxide grows on the metal surface, with the thickness of the film increasing as the electrolysis continues. Competition between the growth of the anodic film and dissolution of the oxide by the acidic electrolyte creates a film having pores which extend from the external film surface inwardly towards the metal article. However, the innermost ends of the pores are always separated from the metal surface by a very thin barrier layer of dense imperforate anodic oxide. If a non-porous anodic film is desired, the anodization can be carried out in a less acidic electrolyte, but only very thin films can be produced in this way depending on the voltage used for the anodization procedure, so the formation of porous films is more usual.
Articles anodized in this way have surfaces which range from grey (i.e. the colour of the underlying metal, generally referred to hereinafter as "colourless" or "clear") to white in appearance depending on the thickness of the oxide film, but various procedures have been developed to colour the anodic films in order to improve the appeal of the articles to the eye. These range from the so-called ANOLOK (trademark of ALCAN ALUMINUM LTD) processes, which involve the electrolytic deposition of a metal (inorganic pigment) into the pores, to the use of dies or organic pigments to cause staining of the anodic film.
While these colouring procedures have been applied successfully for many purposes, they suffer from certain disadvantages. For example, articles coloured by the ANOLOK procedures (as disclosed in our prior U.S. Pat. Nos. 4,066,816 of Jan. 3, 1978 and 4,310,586 of Jan. 12, 1982, both to Sheasby et. al.) may exhibit lack of colour uniformity and the procedure may be difficult to control. Articles coloured by organic pigments and the like exhibit fading when exposed to UV light, and have therefore not been used extensively in exterior (e.g. architectural or automotive) applications.
Moreover, when it is desired to produce coloured patterns on the surfaces of anodized articles, resort has generally been made to the use of adhering masks and the like to cover certain areas of the surface while other areas are subjected to a colouring treatment. The masks then have to be removed and, if desired, further areas masked so that the uncoloured areas can themselves be coloured. This is not only a complex and expensive procedure, it also requires the use of masking materials and solvents that may cause environmental problems when disposed of.
In our prior European patent application Ser. No. 90303069.0 filed on Mar. 22, 1990 and published under Publication No. 0 389 274 A2 on Sep. 26, 1990, a method is described of producing optical interference structures incorporating porous anodic films in which interference colours are generated by the inclusion of semi-reflective layers into the films by electrodeposition and the like. It is disclosed that the deposits may be made more resistant to leaching by replacing the deposited metal with a more noble metal which is much more resistant to corrosion. However, the method is used only for producing films of uniform colour throughout, rather than patterned films. If patterns are required, masking techniques must again be employed.
It is therefore an object of the invention to provide a process which can result in the production of patterned anodic films which are less susceptible to colour loss (fading) or loss of colour uniformity, while providing a good range of colours.
It is also an object, at least of preferred forms of the invention, to provide a process which can produce coloured patterns on anodized surfaces without resort to the use of masks temporarily adhered to the anodized surfaces.
Yet another object of the invention is to provide a process for producing coloured patterns on an anodized surface by a procedure which generates colours at least partially by interference effects.